Add a helper function to the swapout daemon's deactivation code.

vm_swapout_object_deactivate_pages() is renamed to
vm_swapout_object_deactivate(), and the loop body is moved into the new
vm_swapout_object_deactivate_page(). This makes the code a bit easier
to follow and is in preparation for some functional changes.

Reviewed by: jeff, kib
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D22651